When broaching a series of aligned bearing bores, such as the crankshaft bearing bores of an engine block, care must be taken to insure that the bores are cut true, that is, that they are round and in line. The problem is particularly troublesome when the bearings are formed of different materials having different machining characteristics. As an example, the crankshaft bearings of a cast aluminum engine block may have ductile iron bearing caps, so that one-half of each bearing is aluminum and one-half is iron. The aluminum portion of each bearing is softer and gummier and prone to tearing, whereas the ductile iron bearing caps are abrasive and have a higher cutting force constant. This causes the broach to stray off line and push into the softer aluminum material. As a result, the broach may cut holes that are off line and egg-shaped rather than round. Also, the forces generated by the broaching process may cause the bearing caps to deflect during broaching, causing an out-of-round condition in the final bores.
In accordance with the present invention, as shown in the embodiment about to be described, a plurality of broach guides are aligned with the bores to be broached. When the broach is moved through the bores, the path of the broach is controlled by the guides. Preferably, the guides are positioned in the spaces between the bores.
More specifically, the bores are broached in two stages. In the first stage, a broach having arcuately spaced, longitudinal grooves which cross the cutting teeth and divide them into arcuate cutting segments is employed. The broach guides have a plurality of ribs arranged in the same arcuately spaced relation as the grooves about an axis coinciding with the aligned axes of the bores. When the broach is advanced through the bores, the ribs are slidably received in the grooves to guide the broach so that the cutting teeth segments broach arcuately spaced sectors of the bores but leave unbroached ridges therebetween. Thereafter, in a second stage, a second broach broaches the ridges left in the first stage.
Preferably, the second broach has pilot teeth for guiding on the arcuately spaced sectors broached in the first stage. These pilot teeth have reliefs to clear the ridges. Cutting teeth are provided which are dimensioned to clear the sectors broached in the first stage and to broach the ridges left in the first stage.
One object of this invention is to provide broaches, and a method of broaching, having the foregoing features.
Another object is to provide broaches which are rugged and durable in use.
Other objects, features and advantages of the invention will become more apparent as the following description proceeds, especially when considered with the accompanying drawings.